The invention relates to a release agent, which is suitable for the production of polyurethane foam articles in molds and which can be used in processes for producing such articles.
Polyurethane foam articles are produced in molds and the most varied prior art processes can be used for this process. The Expert is aware of a large number of possible variants both with regard to the foam-forming components which can be used and with regard to the equipment and molds which can be used. Generally the mold is filled either in its open or closed state through an injection channel with the polyol and isocyanate components mixed by means of a mixing head. Apart from the sought product and its desired properties, the composition of the components is decided by whether a single-stage process (direct process) or a two-stage process (prepolymer process or semiprepolymer process) is to be used. All these different possible procedures are described in detail in the literature, so no detailed description need be given thereof. Reference is for example made to "Integral-schaumstoffe", Piechota and Rohr, 1975; Kunststoff Handbuch, Vol. VII, "Polyurethane", 1966; "Schaumkunststoffe", published by Fachverband Schaumkunststoffe in GKV, 1976; and Wittfoht, Kunststoff-Technisches Worterbuch, part 3.
For the purpose of an easier and cleaner removal of the polymerized foam articles it is necessary to treat the mold with a release agent (mold release agent) before it is filled. For this purpose, the mold surfaces are uniformly coated with a release agent in order to produce a release film. Apart from leading to good mold release characteristics, in certain cases the release agent must also be able to influence the surface characteristics of the finished polyurethane articles. Thus, as a result of certain additives, e.g. in the cold soft foam sector (seats, headrests, armrests, etc. in cars) an open-cell structure is produced and consequently the necessary "breathability" of the foam article.
Conventional release agents consist of organic solvents and release-effective substances dispersed therein, which are referred to in their entirety as solids. As a function of the intended use, the solid proportion consisting of waxes, greases (long chain hydrocarbons), fats (triglycerides such as beef tallow), silicone compounds, plasticizers, stabilizers, accelerators, etc. varies between 1 and 8% by weight. The proponderant proportions of solvents in the parting compound, such as e.g. Frigen, methylene chloride, trichloroethylene, perchloroethylene, gasolines and high-boiling petroleum hydrocarbons serve as carriers, so that the release effective substances can be applied to the mold surfaces in the form of a uniform release film. Whereas halogenated hydrocarbons exclusively fulfil a vehicle function, the gasoline proportion leads to a uniform flow of the release agent film, which also has a positive influence on the release action.
The solids composition, solids proportion and solvent combination are decided as a function of the mold temperature, the airing time (time between release agent application and component injection) and the particular foam system (integral, rigid integral or soft cold foam). As a result the most varied articles with very varying characteristics can be produced. Thus, for example, integral foam can be used for producing steering wheels for cars, where special requirements are made on the surface (uniformity and good gripping properties). During the production of the integral foam, the mold temperature is generally between 30 and 50.degree. C. However, in the RIM process (RIM foam), frequently used for the production of polished moldings, the mold temperature is 50.degree. to 70.degree. C. Similar temperatures, namely in the range 40.degree. to 70.degree. C., are exhibited by molds when producing articles from cold soft foam such as seats, headrests and arm rests for cars. The mold temperature is generally 30.degree. to 50.degree. C. when producing rigid foam articles, such as car brackets, window shutters, refrigerator components, furniture parts and chairs.
On applying the release agent, which generally involves using an airless spraying device, the solvents largely evaporate as an azeotropic mixture from the mold surface heated to between 25.degree. and 70.degree. C. as a function of the particular foam system. Together with the overspray, these vapours are removed from the working area by corresponding suction equipment. Apart from the loss of valuable raw materials, these pollutant emissions are prejudicial to the environment. Recycling processes would be conceivable with adsorption installations, but a serious problem is posed by the solids in finely divided form, which cannot be eliminated by means of filters or water walls. The adsorption coatings immediately covered with wax and grease would have to be regenerated in very short time, which would involve a large amount of labour and high costs.
It is known that the aforementioned problems can be avoided in a few fields (hot foam, modified integral foam in the case of underfoaming processes) through the use of a release agent, whose organic solvents are largely replaced by water. These are wax dispersions or emulsions, whose use has been very limited up to now with regard to the surface characteristics of the foam articles. In addition, long airing times are required in connection with the aforementioned foam production processes and the mold temperatures employed, so that their use in industrial mass production is uninteresting from the economic standpoint. Apart from the unfavourable evaporation behaviour and inadequate film formation, the main reason for the above limitation in connection with aqueous release agent systems is the competing reaction between water and isocyanate groups (R-N.dbd.C.dbd.O+H.sub.2 O.fwdarw.R-NH.sub.2 +CO.sub.2), which runs parallel to the polyol - isocyanate reaction and liberates carbon dioxide, whilst partially shifting the precisely defined polyol/isocyanate ratio. This leads to foam results in the form of discolourations, bubbles, voids (blisters) and even partial foam collapse. Moreover, on injecting the foam-forming components by means of the injection channel there is an intimate mixing of the components with the release agent in said channel. Here shrinkage phenomena can be observed as another disturbance.
In addition, largely aqueous release agents are known, but, apart from water, they contain considerable proportions of low-boiling, water-soluble alcohols, ketones, esters, etc. as evaporation accelerators. Although these components reduce the long evaporation times required with aqueous release agents, they do not prevent the aforementioned reaction between water and isocyanate. Moreover, these water-soluble evaporation accelerators are still highly prejudicial to the environment.
As a result of the aforementioned disadvantages, aqueous release agents of the aforementioned types, which generally contain considerable solubilizer proportions have not as yet been adopted for industrial use.